1. Field of Invention
This invention relates to a processing method of a printed wiring board and in particular to a processing method for VIA hole processing in a multilayer wiring board.
2. Related Art
In order to form a hole (VIA hole) for electrically connecting conductor layers above and below the insulating layer of a printed wiring board in the insulating layer by laser processing so as to expose the lower conductor layer to the hole bottom, the hole must be made only in the insulating layer and the residue of the insulating layer must not be left on the lower conductor layer.
Unexamined Japanese Patent Publication Hei. 10-322034 discloses that a substance whose color is changed or which emits light by ultraviolet rays or electron rays is added to and mixed with an insulating layer, ultraviolet rays or electron rays are applied to a hole bottom after hole processing and chemical etching, and whether or not the hole processing condition is judged from the light emission area of the hole bottom face. In this case, however, whether or not the hole bottom reaches the lower conductor layer during laser processing cannot be detected.
On the other hand, JP-A-10-85976 discloses that the reflectivity factor difference between an insulating layer and a lower conductor layer is used to measure the reflection of laser strength of laser, thereby detecting a hole bottom reaching the lower conductor layer. In this case, the hole bottom reaching the lower conductor layer can be detected during laser processing, so that processing laser is controlled based on the detection result, whereby an appropriate hole can be made only in the insulating layer. It also discloses that laser is additionally applied for removing the residue of resin on the surface of lower conductor layer because the reflectivity and thermal conductivity of the lower conductor layer are high and thus the temperature of the resin on the surface of lower conductor layer surface resin does not rise.
However, since the reflection of laser of laser has a high directivity, the detection direction of the reflection of laser is limited and in addition, the detection direction of the reflected laser is affected by the roughness and inclination of the hole bottom face and to use the same material as the lower conductor layer for the upper conductor layer of the insulating layer, it is difficult to carry out inspection with accuracy.
It is therefore an object of the invention to provide a processing method of a printed wiring board capable of precisely and easily detecting a hole drilling an insulating layer.
To the end, according to the invention, there is provided a processing method of a printed wiring board wherein in order to produce a hole for electrically connecting an upper conductor layer of an insulating layer of a printed wiring board and a lower conductor layer of the insulating layer into the insulating layer so as to expose the lower conductor layer to a hole bottom, when laser processing is executed for forming the hole in the insulating layer using the printed wiring board, comprising a treatment layer, being placed between the lower conductor layer and the insulating layer, for emitting an electromagnetic wave having a wavelength different from that of processing laser during the laser processing, change in a signal emitted from the treatment layer of the printed wiring board is measured to determine the remaining state of the insulating layer. The electromagnetic wave emitted due to laser processing from the treatment layer placed between the lower conductor layer and the insulating layer rather than the reflection of laser of laser is used, so that the hole drilling the insulating layer can be detected precisely.
As the treatment layer, preferably the printed wiring board can comprise a treatment layer provided by oxidizing a surface of the lower conductor layer, a treatment layer provided by placing a conductive material for emitting an electromagnetic wave by laser processing on a surface of the lower conductor layer, a treatment layer provided by placing a resin containing a component having a strong strength of emitting an electromagnetic wave by laser processing on a surface of the lower conductor layer, or the like.
Inspection illumination may be applied to the hole and at the same time as signal change of the electromagnetic wave emitted from the treatment layer during the laser processing is detected, the strength of the reflection of laser from the lower conductor layer or the emitted light from the treatment layer may be measured to determine the remaining state of the insulating layer.
The printed wiring board may comprise a resin for shielding the electromagnetic wave emitted from the treatment layer as the insulating layer.
To determine the remaining resin thickness on the lower conductor layer, the peak strength of the electromagnetic wave emitted from the treatment layer during the laser processing can be measured and the measured peak strength can be compared with a preset reference value, thereby detecting the remaining resin thickness on the lower conductor layer; the remaining resin thickness on the lower conductor layer can be detected based on the fact that the peak strength of the electromagnetic wave emitted from the treatment layer exceeds a preset upper limit reference value and then is decreased and falls below a preset lower limit reference value; the reflection of laser of processing laser is measured at the same time as the electromagnetic wave emitted from the treatment layer is measured, and the remaining resin thickness on the lower conductor layer can be detected based on the fact that the reflection of laser strength of laser becomes equal to or greater than a reference value of laser reflection and that the electromagnetic wave strength becomes equal to or less than an emission strength reference value; the remaining resin thickness on the lower conductor layer can be detected based on the fact that the signal waveform of the electromagnetic wave emitted from the treatment layer is attenuated before the termination of laser irradiation and a peak is detected; or the remaining resin thickness on the lower conductor layer can be detected by measuring the strength of the electromagnetic wave emitted from the treatment layer, calculating the integration value of the emission strength for each hole, and comparing the integration value with a preset reference value. However, the invention is not limited to the methods.
Preferably, only the electromagnetic waves having a wavelength ranging from 500 nm to 2000 nm, of those emitted from the treatment layer during laser processing are measured; however, any other wavelength band may be adopted depending on the electromagnetic wave emitted from the treatment layer.
Further, preferably the thickness of the treatment layer is set equal to or less than the thickness of the resin that can be removed by executing chemical etching of a next process and when a signal of the electromagnetic wave emitted from the treatment layer is measured, the laser processing is stopped.
Preferably, as the treatment layer, a treatment layer with the emission strength of the electromagnetic wave emitted during the laser processing being twice or more as large as that of the electromagnetic wave emitted by the insulating layer during the laser processing of the insulating layer is used. In this case, the emission strength on a specific wavelength may be twice or more as large as that on the specific wavelength of the electromagnetic wave emitted from the insulating layer.
An electromagnetic wave may be measured on the wavelength at which the maximum strength of the electromagnetic wave emitted by the treatment layer during the laser processing thereof is provided and may be measured on the wavelength at which the strength of the electromagnetic wave emitted by the treatment layer during the laser processing thereof is small and the strength of the electromagnetic wave emitted by the insulating layer during the laser processing thereof is large and the remaining state of the insulating layer may be determined from the electromagnetic wave strength on both the wavelengths, or the electromagnetic wave emitted by the treatment layer during the laser processing thereof may be measured on a plurality of wavelengths and the remaining state of the insulating layer may be determined from the peak strength ratio on the wavelengths.
To thus detect the electromagnetic waves of a plurality of wavelengths, preferably the electromagnetic wave emitted during the laser processing is split by a spectroscope and the post-split electromagnetic waves are detected using two or more detectors.
If a dichroic mirror is placed between a lens for condensing processing laser to a processing position and the printed wiring board and the electromagnetic wave emitted during the laser processing is taken out by the dichroic mirror to the outside of the optical axis of laser and is guided into a detector, the electromagnetic wave can be detected without the effect of the material of the lens.
At this time, if a photosensor array is used as the detector or the condensing lens is placed between the dichroic mirror and the detector, an extremely preferred result can be provided when laser the process is executed in laser beam scanning.
To execute laser processing for making a hole in an insulating layer, change in a signal emitted from the treatment layer of the printed wiring board may be measured to determine the remaining state of the insulating layer and when the remaining thickness of the insulating layer is larger than a setup value, an additional pulse of processing laser may be applied.